EP-A 0 294 323 discloses a yarn brake wherein the yarn is guided between a rigid braking surface (the top of the bottom brake member) and a flexible lamella movable by electromagnets. The lamella is in the form of a thin ferromagnetic metal band or strip or the like. It is of reduced mass and therefore reacts rapidly in an advantageous manner to brief on-times of the electromagnets. Since the lamella is thin, a number of electromagnets are required and experience indicates that at least five are needed; they are disposed in a row in the direction of yarn movement below the rigid braking surface. This has the further advantage that braking occurs at a number of places with relatively reduced surface pressures and therefore in a manner not damaging the yarn. A disadvantage, on the other hand, is that this yarn brake is a relatively long construction and therefore takes up considerable space as compared with other known yarn brakes, in which the yarn is clamped in spots at just one or possibly two places. If the magnets are arranged differently, for example, in a double row, yarn brake length can be reduced, but at the cost of increased yarn brake width.
It has been found in practice that the known yarn brake having an electromagnetically operated brake lamella has a further disadvantage, in that fly evolved from the yarn accumulates on the braking surfaces, more particularly on the longitudinal edges of the lamella. Of course, the adhering fibers enlarge the brake gap or clearance. Since the braking force decreases rapidly with increasing clearance, braking is greatly impaired by the fly deposits. It is therefore necessary to remove the disturbing fibers either continuously or intermittently. This requirement leads to a possibility of releasing the brake far enough for the fibers to be cleared readily from the brake gap, for example, by blowing or sucking.
As is already known, (EP-A 0 294 323) to facilitate release of the brake, individual electromagnets (also called magnet windings) are devised as release windings. In this case, laminar permanent magnets are provided on the brake lamella opposite the release windings and on the side remote from the yarn. With the braking magnets off and if the release windings are polarised appropriately, the brake lamella can be completely disengaged--i.e., the yarn brake can be released. However, the additional use of release windings entails the disadvantage that the yarn brake takes up even more space.
Other disadvantages concern yarn thickness. In the case of very thin yarns, residual magnetism causes the brake lamella to stick to the bottom brake member and in picking mechanisms the lamella then produces detrimental tensions in the weft yarn at the start of a pick. The disadvantage can be obviated by the possibility of opening the brake gap by means of release windings. A problem with thick yarns is that the brake gap is relatively wide and so the magnetic braking forces are relatively weak. This disadvantage can be obviated by force amplification--i.e., by using instead of just a single brake lamella a number of ferromagnetic lamellae disposed one above another.
It is the object of the invention to provide for the yarn brake hereinbefore described for providing force amplification for both the braking and the release electromagnets, improvements being made with respect to the disadvantages mentioned with particular emphasis on minimum impairment of lamella flexibility.
To amplify the braking force and the release force according to the invention, soft ferromagnetic metal laminae are disposed between the brake lamella and the brake cover. The laminae form a segmented layer. The discrete segments, arranged in accordance with the operative zones of the electromagnets, are interconnected; however, the connections between them must be flexible so that the flexibility of the segmented layer corresponds to the flexibility of the brake lamella. The segments can also be connected thereto, in which case the contact zones providing the connections should be very small to ensure minimum impairment of lamella flexibility. At least individual segments must be connected to the lamella so that when electromagnets in the brake cover act to produce a release of the brake, the lamella is compelled to disengage from the bottom braking surface.